Sync periodic_task from oslo-incubator

Sync periodic_task from oslo-incubator

This is more or less to get commit c63fd5a from oslo into the core
projects which have several periodic tasks.  Neutron has periodic tasks
for L3, load balancing and metering agents to sync up state with the
server and most don't have specific spacing values set which can lead to
non-deterministic spacing of when the tasks run.

Note that this does not include the gettextutils and log dependencies
since there are not functional changes in those modules needed for the
periodic_task changes synced in *and* more importantly, the changes
to gettextutils and log require pervasive changes to neutron which
should happen when neutron integrates with the oslo-i18n library for
blueprint i18n--messages.

Further note that this does not include jsonutils due to some
issues introduced with a change for python 2.6 that impacts how strings
are encoded with simplejson. The details for that issue are in bug
1314129.  The jsonutils changes are not related to the periodic_task
changes being synced in so the dependency is not functionally required.

The LbaasAgentManager extends PeriodicTasks but wasn't calling the
parent class init function, which was causing failures since commit
47c9d60 changed PeriodicTasks to init _periodic_last_run, so also
fixed that here.

Changes:

c63fd5a Make unspecified periodic spaced tasks run on default interval
f0dd798 Remove rendundant parentheses of cfg help strings
fcf517d Update oslo log messages with translation domains
051b9f3 Refactor unnecessary arithmetic ops in periodic_task
674cdaf Refactor if logic in periodic_task
b6b82c5 Use timestamp in periodic tasks
47c9d60 Don't share periodic_task instance data in a class attr
8b2b0b7 Use hacking import_exceptions for gettextutils._

Related-Bug: #1319232

Change-Id: Ib23e33326129be0dd952efde263f381d5aa2457f